It has been known for some time to prepare metal surfaces for subsequent painting by applying a phosphate coating so as to attain improved corrosion protection and better paint adhesion. The primarily used base coats for electric immersion coating are zinc phosphate coatings. To produce these, aqueous acid coating solutions are usually used on a base of mono-zinc-phosphate to which chlorate, nitrite, nitrate, organic nitro-compounds or mixtures thereof are added as catalyzers. The resulting coatings are, however, not satisfactory for subsequent painting because of their thickness and coarse crystallinity. As thin and fine grained coatings are desired for pre-treatment prior to electric immersion painting, various organic or inorganic chelating agents, such as citric acid, tartaric acid, malonic acid, polyphosphoric acid, glycerol phosphoric acid, ethylene-diamintatra acetic acid, nitrilotri-acetic acid, or their salts, are often added to the solutions. This, however, results often in problems with controlling the solutions; also, the coatings frequently do not meet the required standards. See U.S. Pat. Nos. 3,523,043; 3,597,283; 3,617,393; 3,647,568.
A disadvantage of the coatings with the known solutions on the base of mono-zinc-phosphate for subsequent electric immersion coating consists particularly in the fact that a considerable part of the phosphate coating is separated during the painting process and is absorbed by the paint-film with detrimental results.
German Published Application No. P 22 32 067 avoids these disadvantages, in that it provides treatment solutions in which the zinc portion in relation to the phosphations is considerably lower than in the customary solutions on a base of mono-zinc-phosphate. The treatment results in improved thin and even phosphate coatings on metal surfaces, particularly iron and steel, with good adhesive strength and durability, and are particularly well suited for subsequent electric immersion coating. The phosphate coatings provide excellent corrosion protection and are separated during painting to a much lesser degree than the coatings from the heretofore known solutions. The described solutions are easily controlled, as chelating agents need not be added in order to attain the desired thin and evenly fine coatings. The ratio of Zn:PO.sub.4 is easily controlled. The solutions contain the usual amounts of PO.sub.4, e.g. ab. 5-20 g/l, but considerably less zinc.